The present invention relates to a power steering device for obtaining a power assist force (auxiliary steering force) by actuating a power cylinder in accordance with the steering operation of a steering wheel and, more particularly, to a power steering device having a steering damper for attenuating an impact generated upon input of a reverse force such as kickback from the steering wheel.
Generally, a hydraulic power steering device has a flow channel selector valve for switching the flow direction of a pressure oil to a power cylinder in response to the steering operation of a steering wheel. More specifically, the flow direction of the pressure oil is switched from the left to the right chamber of the power cylinder when leftward steering is to be performed, and from the left to right chamber of the power cylinder when rightward steering is to be performed, thereby obtaining a power assist force for operating the steering wheel.
In an automobile mounted with such a power steering device, various types of automobiles each in which the power steering device is provided with a steering damper are proposed in, e.g., Japanese Patent Laid-Open Nos. 58-139867, 61-12468, and 62-43367.
With a steering damper, during travel of an automobile, for example, a force that operates the steering wheel from the steering wheel side due to the unevenness and obstacles of the traveling road surface attenuates and moderates an impact generated when a reverse force serving as so-called kickback is input, so an abrupt impact is not transmitted to the steering wheel.
Among conventional steering dampers, the most general one has a restrictor or variable restrictor provided to each of pressure oil paths (right and left cylinder paths) connecting the flow channel selector valve of the power steering device and the right and left chambers, respectively, of the power cylinder. When a force such as kickback sent from the steering wheel acts on the power cylinder, the restrictor or variable restrictor resists against the flow of pressure oil reversely discharged from the power cylinder upon movement of the piston.
When, however, a forward force is input upon steering operation of the steering wheel, such a restrictor or variable restrictor resists against the flow of pressure oil from the pump and the return flow of pressure oil to the tank. Accordingly, the restrictor or variable restrictor does not respond quickly to the actuation of the power cylinder.
Therefore, Japanese Patent Laid-Open No. 62-43367 described above proposes a power steering device in which the restrictors, in the right and left cylinder paths, described above are formed of variable restrictor valves and check valves are connected in parallel to the respective variable restrictor valves. When a forward power is input upon steering operation, the check valve of the supply cylinder path is opened, so that the pressure oil can be sufficiently sent from the pump to the power cylinder.
In this prior art, the opening degree of each variable restrictor valve is variable and depends on the oscillation frequency in the oil chamber formed in the steering damper main body.
Even with the presence of the check valves, the return flow from the power cylinder cannot but pass through the variable restrictor valve. The variable restrictor valve and check valve serve as resistors to decrease the response speed of the power cylinder.
In the conventional steering damper described above, the variable restrictor valve for attenuating the impact generated upon input of a reverse force such as kickback from the steering wheel, and the variable restrictor valve for the forward force input upon steering operation of the steering force share the same return channel. Even when a sufficient amount of pressure oil is needed as in quick steering operation, supply of the pressure oil is sometimes interfered with.
More specifically, if the variable restrictor valves are provided considering seriously the performance for attenuating the impact generated upon input of a reverse force such as kickback, a sufficient return flow from the power cylinder cannot be obtained when a forward force is input accompanying steering operation, and a necessary amount of pressure oil cannot be supplied to the power cylinder.
If supply of the pressure oil is insufficient, operation of the power cylinder is delayed due to the insufficient pressure oil supply. The steering follow-up performance of the steering wheel accordingly becomes insufficient to cause so-called dragging in operation of the steering wheel especially in quick steering operation, making the steering operation unsmooth.
In the power steering device, in order to enable steering wheel operation free from dragging upon input of a forward force and to improve the response speed of the power cylinder, thereby improving the steering follow-up performance, the path diameters of the right and left cylinder paths must be increased. Then, however, a problem occurs when a reverse force such as kickback described above is input from the steering wheel. A countermeasure that can meet these contradictory demands is sought for.